Recovery of Gold from Basic Solution using Electroless Displacement Deposition onto Si Powder

- pH Dependence and Recovery from Leaching Solution-

Abstract

Recovery of noble metals from scrap electronics（urban mines）and industrial waste including plating solutions has attracted keen attention. We recently developed an efficient method of recovering noble metals through electroless displacement deposition onto silicon powder from acidic solutions. For this study, we applied this method to gold recovery from a basic solution. Commercially available silicon powder was added to a simple HAuCl₄ aqueous solution with pH adjusted using HCl and NaOH. Gold was recovered only slightly from the solution with pH lower than 8, but the gold recovery continued and the recovery rate increased from a solution with pH higher than 9. In the low-pH solution, the silicon oxide layer inhibits displacement deposition of gold onto silicon powder. In the high-pH solution, as the silicon oxide layer dissolves, the local anodic reaction of silicon dissolution continues. Results show that the gold recovery rate increased with the amount of silicon powder and solution temperature. At solution pH of 13, gold of 0.001 mol dm^－3 was 100% recovered by 30 min treatment with 0.5 mol dm^－3 of silicon powder at 298 K. From a model leaching solution including Na₃Au（SO₃）₂, CuSO₄,（NH₄）₂S₂O₃, and NH₄OH at pH of 9.3 and a waste of displacement gold plating solution including KAu（CN）₂ at pH of 12.4, gold was recovered efficiently merely by addition of silicon powder. This method is anticipated as a simple, safe, and efficient gold recovery method.